It's hard to choose, really, as I enjoy pretty much all of the steps (except for spell-checking - spell-checking is a BORE), but I think what I enjoy most is the planning and the sketching. Planning is nice because I get to try out possibilities, tinker with things, make stuff up, etc., - and sketching, because that's when the story turns from a potential thing in my head, to a real thing existing in the real world.
It's just neat to see it turn up on the page, is all.